1. Field of the Invention
The present invention relates to driving a liquid crystal display device, and more particularly, to a driving unit for a LCD device.
2. Description of the Background Art
In general, a liquid crystal display (LCD) device includes a liquid crystal display panel formed by attaching a thin film transistor array substrate to a color filter substrate with a uniform gap therebetween. A liquid crystal material fills the gap between the two attached substrates, which face each other. The LCD panel also includes a data driving unit for providing image data to the LCD panel and a gate driving unit for providing a scan signal to the LCD panel.
The thin film transistor array substrate has a plurality of data lines arranged at regular intervals in a first direction and a plurality of gate lines arranged at regular intervals in a second direction, which is orthogonal to the first direction. Pixels are defined by the gate lines and the data lines. Each pixel is provided with a switching device.
A pixel electrode and a common electrode are provided at the inner surfaces of the thin film transistor array substrate and the color filter substrate that face each others. The liquid crystal material between the thin film transistor array substrate and the color filter substrate is driven by a voltage difference between the pixel electrode and the common electrode. The brightness of an image displayed on the LCD panel changes in accordance to the voltage of an image data applied to the pixel electrode. The pixels are electrically connected to the data lines through the thin film transistors in response to signals on the gate lines. Accordingly, if the scan signal from the gate driving unit are sequentially supplied to the gate lines, the switching device of the pixels connected to the gate lines to which the scan signal are supplied are turned-on, and the data driving unit provides the pixels with image data through the data lines.
To provide viewers with a high quality image, digital image data is used so that the image data can be easily compressed and an image having a high color content and high resolution can be implemented. The digital image data can be applied to the LCD device using a ramp signal sampling method or a digital/analog converting method. A gamma correction can be more easily done on a pixel in the ramp sampling method by controlling the ramp signals as compared to the digital/analog converting method. Further, a gamma correction in the ramp sampling method does not require an analog circuit.
In the ramp signal sampling method, analog ramp signals are sampled based upon digital image data, and the sampled signals are supplied to the pixels of the LCD device. The ramp signal is a normalized waveform corresponding to the brightness change of an image according to a voltage potential of the image data applied to the LCD device. A related art unit for driving a LCD device using the ramp signal sampling method will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is an exemplary view of a unit for driving a LCD device in which the related art ramp signal sampling method is applied. As shown in FIG. 1, the related art driving unit of the LCD device includes an input register unit 12 for sequentially sampling and storing N-bit digital image data (DATA[R, G and B]) according to control signals (CS1 to CS3) of a shift register unit 10; a counter unit 20 for outputting control signals (C11 to C13) by individually counting the N-bit digital image data (DATA[R, G and B]) input from the input register unit 12 by a load signal (LOAD) and a clock signal (CLK); and switching units (SW1, SW2 and SW3) for sampling ramp signals (R_RAMP, G_RAMP and B-RAMP) respectively and outputting them to data lines (D1, D2 and D3). The operation of the related art unit of driving an LCD device having such construction will be described in detail with reference to a view showing waveforms in FIG. 2.
FIG. 2 shows graphs of ramp signals, control signals of the counter unit (shown in FIG. 1), and voltages applied to the data lines (shown in FIG. 1). First, the input register unit 12 (shown in FIG. 1) sequentially samples and store N-bit digital image data (DATA[R, G and B]) according to the control signals (CS1 to SC3) of the shift register unit 10. Then, the counter unit 20 receives the N-bit digital image data (DAT[R, G and B]) from the input register 10 by the load signal (LOAD), individually counts each bit of the N-bit digital image data (DATA[R, G and B]) by the clock signal CLK and outputs the control signals (C11 to C13). The N-bit digital image data (DATA[R, G and B]) input from the input register 10 by the load signal is stored in a storage latch formed in the counter unit 20.
In the case that the 6-bit digital image data (DATA[R]) is input into the counter unit 20 as ‘000100’, the counter unit 20 is driven by the clock signal (CLK) and counts the digital image data until ‘000000’ becomes ‘000100’, and outputs the control signal (C11), which is at a high potential during the counting of the image data and transitions to a low potential when the counting is completed. In the case the 6-bit digital image data (DATA[R, G and B]) is provided as ‘100110’, the counter unit 20 is driven by the clock signal (CLK) and counts the digital image data until ‘000000’ becomes ‘100110’, and outputs the control signal 12 as a high potential during the counting of the image data and transitions to a low potential when the counting is completed. In addition, when the 6-bit digital image data (DATA[R, G and B]) is provided as ‘111111’, the counter unit 20 is driven by the clock signal (CLK) and counts the digital image data until ‘000000’ becomes ‘111111’, and outputs the control signal 13 as a high potential during the counting of the image data and transitions to a low potential when the counting is completed.
Meanwhile, the switching units (SW1, SW2 and SW3) receive the control signals (C11, C12 and C13) individually from the counter unit 20 and are turned-on by the high potential control signals (C11, C12 and C13) such that sample waveforms of the ramp signal (R_RAMP, G-RAMP and B_RAMP) are supplied to the data lines (D1, D2 and D3). The highest potential of the sampled waveforms (R_RAMP, G-RAMP and B_RAMP) according to digital information of the N-bit digital image data (DATA[R, G and B]) is set as a pixel voltage and then supplied to the data lines (D1, D2 and D3). The sample waveforms are provided to the pixels of the gate lines turned-on by a scan signal as well as the pixel voltage, which is maintained for one frame.
In the LCD device using the ramp signal sampling method, a gamma correction on the pixels can be easily performed in comparison to the LCD device using a digital/analog converting method. That is, in the LCD device using the digital/analog converting method, the value of a resistance in an analog device has to be precisely adjusted. However, in the LCD device using the ramp signal sampling method, waveforms of a ramp signal supplied to a pixel can be changed such that the gamma correction can be easily performed.
In addition, the LCD device employing the ramp signal sampling method is not largely affected by characteristic differences of transistors as compared to the LCD device employing the digital/analog converting method, which samples the digital image data as an analog form and applies it to the pixels. That is, the LCD device using the digital/analog converting method requires an analog circuit such as an operational amplifier (OP-AMP) for converting a digital signal to an analog signal. Since the operational amplifier is very sensitive to characteristic differences of the transistors, it has a high offset voltage and consumes much power. However, since the LCD device using the ramp signal sampling method samples the ramp signal having an analog form according to the digital image data and applies the sampled ramp signal to the pixel, an LCD device using the ramp signal sampling method does not require the analog circuit such as an operational amplifier and is not largely affected by differences in the characteristics of the transistors.
To display a high-definition image using the related art ramp signal sampling method, a large number of image data have to be processed. Therefore, counting using a related art counter unit for counting the digital image data for each pixel becomes complicated. Moreover, image data in one frame is counted such that the ramp signal is sampled to appropriately determine a pixel voltage to display an image. As the number of bits is increased, the correcting section is decreased. Thus, the ramp signal can be sampled before it reaches a desired level for a particular image data. Accordingly, a poor picture quality can be generated by an improper ramp signal for an image data that is processed late.